1. Field of the Invention
The present invention generally relates to multi-beam scanning apparatuses and, more particularly, to a multi-beam scanning apparatus in which high-quality image is assured by controlling the scan line bow.
2. Description of the Related Art
Conventionally, a single-beam optical scanning apparatus for writing an image by optically scanning a scanned surface with a single deflected beam is widely known as an image forming apparatus for optical printers and the like. Recently, multi-beam optical scanning apparatuses for simultaneously scanning a scanned surface with a plurality of beams for the purpose of improving image writing speed are being developed.
In the single-beam scanning method, a beam emitted by a light source is guided toward a beam deflector such that a chief ray coincides with an optical axis of the optical system. The chief ray of the beam is deflected by the beam deflector on a plane that includes an optical axis of a scanning image-forming optic such as an f.THETA. lens. In the single-beam scanning apparatus, a scan line (a track of a beam spot) is hardly curved. That is, no scan line bow is created.
In the multi-beam scanning method, however, an optical path of some of a plurality of beams traveling from respective light sources to the scanned surface is displaced with respect to an optical axis of the optical system in a direction (hereinafter, referred to as an unfolded sub-scanning direction) corresponding to a direction that matches the sub-scanning direction when the optical path from the light source to the scanned surface is unfolded. The scan line described by the beam displaced with respect to the optical axis is not straight. That is, a scan line bow is created.
FIGS. 1A and 1B show how four beams from respective light sources produce four beam spots providing four scan lines S1-S4 on a scanned surface. The dashed line A indicates a reference scan line produced when a chief ray of a deflected beam is deflected on a plane which includes an optical axis of a scanning image-forming optic and which is parallel with a direction (hereinafter, referred to as an unfolded main-scanning direction) corresponding to a direction that matches the main-scanning direction when the optical path from the light source to the scanned surface is unfolded. The reference scan line A corresponds to the scan line produced in the single-beam scanning arrangement and is therefore free of bow.
In the example shown in FIG. 1A, the scan lines S1-S4 lie side by side in the sub-scanning direction (vertical direction on the paper) such that the scan lines S1 and S2 are symmetrical with the scan lines S4 and S3, respectively, with respect to the reference scan line A. The scan lines S1 and S2 are bowed toward the top of the paper and the scan lines S3 and S4 are bowed toward the bottom of the paper. FIG. 1A shows how two scanning processes B1 and B2 each using four beam spots simultaneously are successively performed. Optical writing is performed such that the scanning processes B1 and B2 are alternately repeated.
Referring to the area I, a small interval (scan line pitch) is created between the scan line S4 in the scanning process B1 and the scan line S1 in the scanning process B2. In the area II, a large scan line pitch is created between the scan lines S2 and S3. As a result, there is produced a periodical variation in density of the optically-written image in the sub-scanning direction, near the center of the extent of main-scanning. Thereby, image quality is degraded.
FIG. 1B shows how a scanning process Cl using four beam spots is followed by a scanning process C2. The scan lines S1-S4 lie side by side in the sub-scanning direction such that the scan lines S1 and S2 are symmetrical with the scan lines S4 and S3, respectively, with respect to the reference scan line A. The scan lines S1-S4 are slightly bowed away from the reference scan line A at two locations along the extent of main-scanning (left-to-right direction on the paper) and are bowed toward the reference line A at the center of the extent of main-scanning. The scan line S1 is bowed with a larger radius of curvature than the scan line S2. The scan line S4 is bowed with a larger radius of curvature than the scan line S3.
Sine the scan line S4 of the scanning process C1 and the scan line S1 of the scanning process C2 are bowed in opposite directions, a large scan line pitch is created in the area III and a small scan line pitch is created in the area IV. As a result, there is produced a variation in density of an optical-written image in the main-scanning direction, at a boundary between two successive scanning processes C1 and C2. Thereby, image quality is degraded.
A variation in scan line pitches occurring in a multi-beam scanning apparatus due to curvature of the scan lines, will be referred to as a differential pitch. As described above, differential pitch causes the quality of the optically-written image to be degraded.
One approach to reduce the differential pitch is disclosed in Japanese Laid-Open Patent Application No. 7-199109. In this approach, a scanning image-forming optic is designed to produce curvature of field. While producing curvature of field reduces differential pitch, it also causes a large variation in beam spot size on a scanned surface depending on image height. The beam spot size becomes relatively large where the image height is relatively large. Thereby, image quality is again degraded.